1. Field of the Invention
The present invention deals with the process of watermarking digital objects such as still images, video, multimedia, and sound.
2. Art Background
Watermarks are methods for identification or authentication, such as watermarked paper showing a logo or design. In the digital arts, watermarking refers to processes by which the watermark, identifying or authenticating data, is embedded in a digital object, or the mark is extracted from a digital object. Watermarking is used in copyright protection as a means of identifying ownership and enabling the detection of copies and derivatives.
In the case of watermarking still images, most published methods use the watermark to manipulate the amplitude of pixel values directly in the pixel domain. For example, U.S. Pat. No. 5,530,751 to Morris embeds a digital watermark in an object by manipulating the amplitude of selected digital samples of the object. This approach is also used in U.S. Pat. No. 5,636,292 to Rhoads. xe2x80x9cProtecting Publicly-Available Images with an Invisible Watermarkxe2x80x9d by Gordon W. Braudaway, IEEE International Conference on Image Processing, Santa Barbara, October 1997 discloses a method of multiplying the amplitude of the image by the watermark. Such systems that vary the amplitude of the image are vulnerable to additive noise, and are also vulnerable to attack. Other systems, such as xe2x80x9cAdaptive Watermarking in the DCT Domainxe2x80x9d by Bo Tao and Bradley Dickinson, IEEE Conference on ASSP, April 1997; xe2x80x9cTowards Robust and Hidden Image Copyright Labelingxe2x80x9d by E. Koch and J. Zhao, Proceedings of 1995 IEEE Workshop on Nonlinear Signal and Image Processing (Neos Marmaras, Halkidiki, Greece, Jun. 20-22, 1995), pp. 452-455; and xe2x80x9cImage Watermarking Using DCT Domain Constraintsxe2x80x9d by A. Bors and I. Pitas, 1996 EEEE International Conference on Image Processing (ICIP""96), Lausanne, Switzerland, vol. III, pp. 231-234, Sep. 16-19, 1996, disclose techniques for embedding watermarks by enforcing constraints on the ordering or differences of transform values, typically block Discrete Cosine Transforms (DCT). While these systems are more resistant to noise, they are not resistant to other operations such as cropping and scaling as they depend on accurate registration with the original image. The method disclosed in xe2x80x9cPhase Watermarking of Digital Imagesxe2x80x9d by J. J. K. O Ruanaidh, W. J. Dowling and F. M. Boland, IEEE International Conference on Image Processing, Vol. III pp. 239-241, Lausanne, Switzerland, September 1996 hides the watermark in the phase of the DCT; the computational complexity and numerical errors involved in this approach are undesirable, and the phase is extremely susceptible to translations of the original image.
As noted, methods that include a watermark by manipulating pixel amplitudes are susceptible to noise and are vulnerable to attempts at removal. If the watermark is highly correlated, such as a company logo, it will often be visible in the watermarked image. The block DCT methods are susceptible to image translations. A watermarking scheme should result in an invisible or imperceptible mark even when the watermark itself is highly correlated, be difficult to intentionally remove, and be robust to common image transformations such as compression including lossy compression, JPEG compression, requantization, contrast enhancement, brightness variation, conversion to gray scale, blurring, and sharpening.
In a first embodiment of the present invention, watermark information is incorporated in a digital object by including the watermark in the argument of a transform of the image. By using the watermark to perform argument modulation of the object in the transform domain, rather than modifying the amplitude of the image, the resulting watermarked image is more robust and resistant to attack. A second embodiment of the present invention discloses an additional method of argument modulation. A third embodiment combines the watermarking process with lossy compression. In all embodiments the argument modulation process results in a loss of information, thus making the watermarked object less vulnerable to intentional attack.